In the manufacturing of a semiconductor device, various types of heat treatment apparatuses are used to perform treatments, such as oxidation, diffusion, CVD (chemical vapor deposition), etc., of a semiconductor wafer as a processing object. A common heat treatment apparatus comprises a heat treatment furnace including a processing chamber having a furnace opening at the bottom and adapted to house and heat-treat semiconductor wafers, a heat insulator disposed such that it surrounds the circumference of the processing chamber, and a heater, provided on the inner peripheral surface of the heat insulator, for heating the wafers in the processing chamber; a lid for closing the furnace opening of the processing chamber; a holding tool, mounted on the lid, for holding the wafers in multiple stages; and a lifting mechanism for raising and lowering the lid. Such a common heat treatment apparatus generally has a vertical structure, extending vertically as a whole, and is installed in a clean room.
These days such a heat treatment apparatus is required to heat-treat large semiconductor wafers having a diameter of 450 mm. When heat-treating 450-mm semiconductor wafers, the wafers need to be arranged at a considerably larger pitch in a heat treatment furnace. Thus, in order to process a certain number of larger wafers at a time, it is necessary to use a heat treatment furnace having a considerably larger overall height. However, because of the restriction of the interior height of a clean room, it may not practically be possible to increase the height of a conventional vertical heat treatment furnace to such an extent.